We are testing a new combination of medicines, to determine if they could be used to treat cystic fibrosis (CF). Subjects with CF who have two copies of the most common mutation (change) found in patients with CF called DF508. CF is caused by a lack of chloride movement in the nose, sinuses, lungs, intestines, pancreas and sweat glands. We are conducting this study to determine the safety of using a combination of two medicines, Phenylbutyrate and Genistein, to improve the ability of the cells lining the nose to regulate movement of salt (chloride) and water in people with CF.